User.findOne(...).exc is not a function

var express = require('express');
var router = express.Router();
var User = require('../models/user');

// GET / profile
router.get('/profile', function (req, res, next) {
  if (!req.session.userId) {
    var err = new Error('You are not authorized to view this page.');
    err.status = 403;
    return next(err);
  }

  User.findById(req.session.userId)
      .exec(function (error, user) {
        if (error) {
          return next(error);
        } else {
          return res.render('profile', { title: 'Profile', name: user.name, favorite: user.favoriteBook });
        }
      });
});

// GET / logins
router.get('/login', function (req, res, next) {
  return res.render('login', { title: 'Log In' });
});

// POST /logins
router.post('/login', function (req, res, next) {
  if (req.body.email && req.body.password) {
    User.authenticate(req.body.email, req.body.password, function (error, user) {
      if (error || !user) {
        var err = new Error('Wrong email or password.');
        err.status = 401;
        return next(err);
      } else {
        req.session.userId = user._id;
        return res.redirect('/profile');
      }
    });
  } else {
    var err = new Error('Email and password are required.');
    err.status = 401;
    return next(err);
  }
});

// GET / Register
router.get('/register', function (req, res, next) {
  return res.render('register', { title: 'Sign Up' });
});

// POST / register
router.post('/register', function (req, res, next) {
  if (req.body.email &&
    req.body.name &&
    req.body.favoriteBook &&
    req.body.password &&
    req.body.confirmPassword) {

    // confirm that user typed same password twice
    if (req.body.password !== req.body.confirmPassword) {
      var err = new Error('Passwords do not match.');
      err.status = 400;
      return next(err);
    }

    // creat object with form input
    var userData = {
      email: req.body.email,
      name: req.body.name,
      favoriteBook: req.body.favoriteBook,
      password: req.body.password,
    };

    // use schema's `create`  method to insert document into mongo
    User.create(userData, function (error, user) {
      if (error) {
        return next(error);
      } else {
        req.session.userId = user._id;
        return res.redirect('/profile');
      }
    });

  } else {
    var err = new Error('All fields required.');
    err.status = 400;
    return next(err);
  }
});

// GET /
router.get('/', function (req, res, next) {
  return res.render('index', { title: 'Home' });
});

// GET /about
router.get('/about', function (req, res, next) {
  return res.render('about', { title: 'About' });
});

// GET /contact
router.get('/contact', function(req, res, next) {
  return res.render('contact', { title: 'Contact' });
});



module.exports = router;```